Amplifier system



Oct. 28, 1947. c. G. sERlGHT AMPLIFIER SYSTEM 2, 1944 2 sheets-sheet 1Filed June 2 7 www m.

oct. 28, 1947.

AMPLIFIER SYSTEM Filed-June 22, 1944 2 Sheets-Sheet Z2` ATTORNEY c. G.sERlGHT I 2,429,775

Patented Oct. 28, 1947 AMPLIFIER SYS TEM Carl G. Seright, Riverton, N.J., assignorto Radio Corporation of America, a corporation of Dela- WareApplication June 22, 1944, Serial No. 541,502

, 11 Claims.

, 1 Y My present invention relates to amplier systems, and moreparticularly to a, stabilized amplier system having a, plurality ofreproducer I output elements.

One of the important objects of my invention is to provide a pluralityof signal transmission channels with a common output circuit subject toload impedance variation; there being employed a novel and eiiectivemethod of minimizing changes in signal output level due to saidimpedance variation.

Another important object of my invention is to provide a method ofstabilizing the audio output level at the common output circuit ofparalleled audio channels of respective lreceivers so that one or morereproducer units can be connected to the common :output circuit tolisten to the output of one or more of the audio channels.

Still another object of my invention is to provide in an amplifiersystem a method of, and means for, utilizing negative and positivefeedback of signal voltage so related as to produce a stabilized signaloutput level regardless of load impedance variation.

Another object of my invention is to provide a plurality of signaltransmission channels, wherein each channel is adapted to be fed bysignals of diierent character, each channel having a degenerativefeedbackpath individual to it, andY a common positive feedback -pathbeing provided for all the channels tojcooperate with the individualdegenerative paths to provide a stabilized signal output level. Y

Other features of my invention will best be understood by reference tothe following description, taken in connection with the drawing, inwhich I have indicated diagrammatically two circuit organizationswhereby my invention may be carried into elect.

In the drawing:

Fig. 1 shows one embodiment of the invention,

Fig. 2 shows a modification. Y

Referring now to Fig. 1 of the accompanying drawing, wherein there isshown only so much of a pair of paralleled signal transmission channelsas is essential to a proper understanding of my invention, it is to beunderstood that the two channels are of substantially similarconstruetion. However, each channel transmits signals of differentcharacter. The amplifiers I. and I of the respective transmissionchannels may have the signal input grids 2 and 2 thereof connected toany desired sources of signals. The signal sources are not shown, sincethey may be of any desired construction. For example, the channels maybe fed from the respective detector loads of separate radio receiversoperating in widely different frequency bands. Merely as a specicillustration of such use, one receiver could be operative in the 0.19 to2 megacycle (me.) band, While the other is operative in the 2 to 18 mc.range. receiver might feed speech signals to one of the grids 2, whilethe demodulator of the other receiver would feed impulse or directionalsignals to the second channel. Again, the sources feeding grids 2 and 2could be separate microphones functioning intermittently orconcurrently. They could readily be electrical pickups ofrecordreproducing equipment.

Assuming, now, that separate receiving systems feed the detectedoutputs, or modulation signals, thereof respectively to saidtransmission channels, there will now be explained the specicconstruction of the channel component elements. Since the elements ofthe pair of channels are similar, reference will be made to the upperone of them. The lower channel components are denoted by the samenumerals, except that a A prime `designation distinguishes thesereference numerals from the corresponding numerals of the specificallydescribed channel. Oficourse, as many signal channels as is desired maybe fed from each demodulator, or as many separate receivers as isdesired may be employed to feed a corresponding number of transmissionchannels having paralleled output circuits,

AAmplilier tubes I and 3, while shown as pentodes, may be of any desiredand suitable types. 'Ihe signal grid 2 of tube I is coupled by condenser4 to its modulation or other low frequency signal source. Cathode 5 isconnected to ground by a bias resistor made up of two sections 6 and 1.Section I is shown as adjustable so as to provide control over the gainof the channel. Condenser 8 shunts resistor 6-1 to bypass alternatingcurrents, and resistor I9 provides a direct current return path for grid2 so as to apply negative bias to thev grid relative to the cathode 5.The screen I0 and plate II of tube I are connected to a point ofsuitable positive potential +B by respective resistors I 2 and I3. Thescreen and plate voltage supply leads are bypassed to ground foralternating currents by condensers I4 and I5 respectively.

I 1. The grid I6 is connected by condenser Il to Y In such case thedemodulator of onethe plate end of resistor I3, while resistor I8returns grid I6 to the grounded end of cathode resistor I9. The lattersupplies negative biasing voltage thereacross for grid I6. Condenser 30is a direct current blocking condenser in the feedback path, The screen20 and plate 2l of the amplifier tube are.v connected to respective endsof the primary winding 22Y of output transformer 23, while the lower endof winding 22 is connected to a suitable positive potential point'` +B.

Negative or degenerative signal voltage feedback is provided by a path25,--23-48, of whichthe resistor I8 is common tothe output and inputcircuits of amplifier tube 3. The portion of the negative feedback pathwhich is connected between plate 2| and grid I6 is enclosed ina dottedrectangle, and comprises resistor 25 and` oondenser 26 connected inseries. The constants of transformer 23 and of the path 25.--2-i3 arechosen so as to-provide. degeneration to the desired extent of thesignals applied to grid I6.

The secondary winding 24 of output transformer 23 has theA upper endthereof connectedv to an output lead 21while the lower end is connectedto ground through a resistor 28. The latter is, unbypassed, and developsthereacross low frequency signal voltage. The connection 29.,A includingdirect, current blocking condenser 30 is made from the upper end ofresistor 28' t0 the cathode end of resistor I9. The polarity. of winding24 is so chosen that path 29-I9 acts as a positive signal voltagefeedback path.v In other Words, this, feedback. pathV contains resistorI9 which is common to the output, and input circuits of tube. 3, and` itfeeds back SignalV voltage in asense. or phase resulting inself-reinforcement., and4 oppositev to that of the negative feedbackpath 25.-26-I.8.

The. output connection 2.11 is common tov the high potential. terminalsof' windings 24' and 24. ofv transformers 23 and', 23,. The lowpotential end of winding 24 is connected by lead 3I to. the ungrounded`end of resistor 28'. Hence, the resistor 23j is commento the outputcircuits. of both channels, andfunctions to provide positivefeedbackvoltage for both. of.V amplifiers 3 and 3', the latter throughcondenser 3D and resistor t9".

The output loadV of both channels comprises one or more signal outletsor jacks adapted to receive respective signal reproducer devices. Thus,the common connection 32 leads from connection 2 1. to a pair ofparalleljacks 33 and s4; and the latter have a. common ground returnconnection. Of course, as many jacks as is desired. may be shuntedacross the common output Circuit, of the twojchannels. The. reproducerdevices` may be headphones, as wouldV be the case in aircraft,reception. where one headphone set might serve a pilot and the othersetv would be that, of. the radio operator. Each ofthe headphones. wouldbe plugged. into respective jacks 33 and 34j in such case, or one 0fthem could be removed.

The function of the negative feedback path 2.5,-2-I8, and` positivefeedback path 28-29--30-13 in eachA channei is to minimize changes insignalA output level due to changes in the load connected to: the commonoutput circuit, and due to cross loading effects between the amplifiers.The receivingv systems feeding the audio channels are operable one at atime, or simultaneously.. Further, changes in the kind or number ofreproducer devices plugged into the outlets or jacks affect the signaloutput level.

Again, rendering either transmission channel inoperative, as by turningoff its filament voltage, substantially alters the load on the othercircuit.

My invention provides a simple method of minimizing changes in thesignal output level resulting from load impedance variations regardlessof the cause` of such variations.

In orderto visualize the functioning of' my invention, consider firstthe effects of the feedbacks atthe load terminals Y--Y. The impedanceseen by the load at Y-Y can be determined by substituting a voltage forthe load, and dividing this voltage by the resulting current.

Let Eo--an alternating voltage applied Y-Y in placel of the load. Eg=thesum of the feedback voltages on grid I6.

:amplification factor of tube 3.

Ri==internal output resistance of tube 3.

Rr=resistance in ohms of positive feedback resistor 23., Y

Nzratio of output transformer 23=secondary turns+prirnary turns.

Azfraction of the` voltage established across Re whichv is. appliedbetween cathode and grid oftube 3y in regenerative phase.

B=fraction of the voltage across the primary 22 of output transformer23` which is applied between grid and cathode in degenerative phase.

Io=current due to voltage Eo.

at terminals The product of the current and. impedance equals theapplied voltage (EnzIoRc). The total voltage acting in the outputcircuit is the sum of. the applied voltage and the feedback voltages,and the impedanceis the sum of the feedback resstanceRr (28) and theplate resistance of the tube reflected through transformer 23':

wherein Es=thevoltage across secondary output transformer 23.

Es= Et- I 0R11.

EgSARFIVi.

The full Ohms law equation can now be set down: Eor-l-nARiIoN-f-(,aBEo-nBIoRF) :IolRF--RPNZ] The assignment of polarities to thefeedback voltage terms in the foregoing expression is made on the basisof their effect as they reappear in the amplifier output at X-X. Thesevoltages are in series with Eo in the output circuit Y-Y, X-X, RF. Ifthey act in concert with Eo to increase Io, they have the same sign asEn. The polarity of the-B term voltage (enclosed in parentheses) for thecondition'RF=0 is the reverse, from a parallel standpoint, of the Eovoltage, since the B voltage isV degenerative. Acting in series with Enthrough XX, Rran'd Y-Y, this voltage therefore assists Eoin productionof Io. The A term voltage is regenerative, if originated by a voltageappearing at X--X, as for example a B term voltage. The A term voltagereappearing at X-X would reinforce the originating voltage. The A termVoltage is produced by IoRF drop in RF. 1f this IORF drop is produced byva voltage Eo applied at Y-Y,

pearing at X-X produced by En applied at Y-Y.

All the voltage terms, therefore, mutually assist in the production ofIo, hence all have the same sign,

Proceeding with the solution:

It is to be remembered that in the foregoing expression, both the A andthe B terms are positive quantities. Losses in the output transformer 23are neglected as they can be made negligible by suitable design.

It can be seen from 1) that increasing either the positive feedback, asby increasing the value of RF (28), or increasing the negative feedback,as by decreasing resistor 25, will result in a reduction of impedance Ropresented by the circuit to the load at Y-Y. The value of Ro can be madepositive, zero, or negative, by suitable choice of values of resistor28, A and B. A lowered value of Ro, obtainable by the use of feedback,is desirable for minimizing output variations due to load changes. Thus,if it is desired to maintain the total output into load jacks 33-34constant as the impedance of this load is changed, values of resistor28, A, and B are selected which result in an Ro value approximating theaverage value of the load.

If it is desired to maintain the output per element of load constant asthe rnumber of similar load elements is varied, Values of 28, A, and Bare chosen which result in an R value approaching zero. The exactcircuit constants necessary to affect the value of Ru in the desiredmanner can either be determined by mathematical procedure of a morerigorous nature than that indicated a circuit such that the impedancepresented byk each amplifier to the yother is not altered as a result ofthe feedback employed in each, when the load plugged into jacksV 33-34has a selected Value.

Consider a voltage applied between points as by means of connections tooutput winding 24' of another amplifier. The impedance seen by thisvoltage consists of two components in parallel. One branch consists ofthe load plugged into jacks 33-34, plus resistance 28. The other branchconsists of the internal output impedance of the tube 3, as modified bythe ratio of transformer 23. When pentode tubes are used, this lattercomponent is made several times the load resistance by suitableadjustment of the transformer ratio in order to obtain maximumundistorted output from the amplifier. Thus, when no feedback isemployed two pentode arnplifiers paralleled as at X-X do not load eachother greatly, but work eiciently into the load resistance.A In mycircuit the same results are obtained while the advantages of feedbackare simultaneously obtained, as set out above. Referring again to pointX-X in the circuit, and assuming positive feedback through 23, 29, 30,I9 equal to the negative feedback through 25-25-48 for a particularvalue of load impedance, it is apparent that since these feedbackeffects are equal and opposite at X-X, the current through the load andalso through transformer winding 24 will be the same as though nofeedback to tube 3 were employed. By making the positive feedback exceedthe negative feedback, the impedance seen by the second amplifierat.X--X, due to the connections to 24, can be increased over the valuewhich would be obtained with no feedback;y This condition might be founddesirable under some conditions; for example, if tubes having lowinternal output impedance were to be used in the amplifier.

The impedance presented by the circuit to another ampliiier paralleledat X--X varies with the load resistance plugged into jacks 33-34 inaccordance with the following relation, derived in a manner similar toEquation 1.

RPN2

In the aforesaid Equation 2 Rs=resistance presented by the circuit atX-X, with feedback, but exclusive of the load branch.

Rr=load resistance plugged into jacks 33-34. Other notations are thesame as given for Equation l, and the A and B terms again are to beconsidered positive quantities.

Equation 2 shows that for a selected value of RL, if the positive andnegative feedbacks are made equal, by a proper choice of values for A,Rrand B, the circuit impedance at X-X is equal to RPN2, which is thevalue obtained with no feedback. The manner in which the impedance atX-X decreases with an increasing value of RL can also be found byEquation 2. A sim,- ilar representation can be made for the impedancepresented by the second amplifier to the first or by a third amplifierto the first and second.

The desired characteristics for cross loading effects between amplifiersare obtained in my circuit by using a positive feedback element which iscommon to all the amplifiers, This element is numbered 28' in theaccompanying drawing, and can either be a resistance, as shown, or areactive or complex impedance if a frequency discrimination is to beimparted to the feedback action. The positive feedback voltage is shownapplied in the cathode circuit, but it might equally well be applied inthe grid circuit, or the plate circuit of the preceding tube, or at someprevious point in the circuit.

The impedance presented to the load at Y-Y by two or more similaramplifiers with their outputs paralleled at X-X can `be found bydividing Rp everywhere it appears in Equation 1 by the number ofamplifiers in parallel. Thus, although the input to the loadfrom one ofthe amplifiers can be rendered largely insensible to the effect ofadding Ya second amplifier in parallel, as set out above, this conditionactually obtains for only oneY value of load impedance. For abnormallyhigh load impedance, the output may be substantially decreased as thesecond (or third) amplifier is connected in parallel, as can bedetermined from Equation 2. For abnormally low load impedance, thereverse effect would be obtained. Nevertheless, the circuit constantscan be so vadjusted as to maintain a more stabilized output conditionthan has heretofore been obtainable With circuits not incorporating thecommon feedback element represented by 28, and the circuit isparticularly useful in cases Where one or more such amplifiers may beused individually .or simultaneously with a common load.

Merely by way of specific illustration the following list of constantsis given for the system of Fig. 1:

Raar-2.2 megohms Riez-1000 Ohms R1=1000 ohms C2s=680 micromicrofaradsC3a=20 microfarads 017:0.01 microfarad Transformer (2.3) impedanceratio=4021 Summing up, then, one aspect of the operation ofthe system ofFig. 1, land assuming the load at points 33, 3L. to be in the form ofheadphones, and for purposes of the present point considering only oneof the amplifiers, we may assume first that vthe load consists of oneheadphone only. If, then, we add a second headphone in parallel with therst, we cut the ohmic resistance of the load in `half so that, lookingupon secondary 2li as a source of alternating current energy, thevoltage drop across the load is reduced and the voltage drop acrosspositive feedback resistor 28 is increased. We thereby attain reducednegative feedback through 25, 26, 18, and increased positive feedbackacross resistor 28, when energy is fed .in 4parallel to `an increasednumber of headphones. Of course, if the headphones were connected inseries instead of in parallel, the action would be reversed in that thenegative feedback would be increased and the positive feedback would bedecreased-as .the number of series-connected headsets was increased.Substituting a positive parallel feedback for the negative parallelfeedback, and a negative series feedback for the ,positive seriesfeedback described elsewhere herein, would likewise reverse the actionobtained.

vAnother circuit arrangement which I have devised is particularlysuitable when the number of amplifiers which may be in operation inparallel is variable, and when the load impedance is variable, vand itis desired to maintain either a constant total output .per amplifierinto the load, or a constant output per amplifier per element of ,loadinto a load consisting of a variable number of elements. Such a circuitis shown in Fig. 2, wherein one (or more) tube (or tubes) provides thecommon positive and negative feedbacks, while additional tubes notemploying feedback within themselves are employed for additionalamplifiers. With .this modified arrangement, the tube incorporating thetwo feedback paths is always left .in the circuit regardless of anyother combination of outputs, and may serve as one of .the amplifiersand also as a variable resistor which is governed by the load impedance,or may fulll the latter function only. The essential features to benoted about this circuit are:

l(A) The impedance presented Aby the output circuit of each of theamplifiers not employing feedback to the other amplifier output circuit,is equal to RPN2, and this is also the impedance presented by thefeedback tube 3 to the other amplifiers when the load has a selectedValue for d which the positive and negative feedback effects are madeequivalent. A typical output pentode has an internal output resistanceof 70,000 ohms, and a rated load resistance of 7000 ohms. Thus, severalsuch tubes can be operated in parallel without serious loss in energyavailable to the load from any of the tubes.

(B) The impedance presented to the other ampliers by the tubeincorporating the feedback is that given by the aforesaid Equation 2.This impedance can be seen rto vary in an inverse manner as the loadimpedance is varied. The feedback tube 3, therefore, alters itsimpedance in a manner tending to use up the excess energy from allsources when the load impedance is increased, and conversely tends toapply additional energy to the load when the load 'impedance is reducedregardless of the original source of the signal or signals.

A preliminary set 0f values for A, RF, and B can be arrived at bysubstituting actual circuit constants in Equations 1 and 2. Finalcircuit values are more easily determined by laboratory experimentalprocedure.

The modification shown in Fig. 2 follows the numbering of Fig. 1. Thefeedback amplifier tube 3 is provided with the positive feedback elementZ3 and the negative feedback path 25-26. It will be seen that theelectrical circuits associated directly with tube 3 are exactly the sameas those in Fig. 1. Tube 3 is provided with the same circuits as in Fig.l, save that the degenerative path 2526 is omitted,'and condense-r '30by-passes cathode resistor i9 to ground. The third amplifier 3 isprovided with circuits exactly similar to those of tube 3. Therefore,the same numerals are used for the circuits of tube 3, except thatdouble prime designations are utilized in the latter case. Leads 40 mayconnect to additional signal amplifier output channels.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that my'invention is by no means limited to the particular organizations shownand described, but that many modifications may be made without departingfrom the scope of my invention.

What I claim is:

-l. In combination with at least two signal transmission channels eachhaving input .terminals adapted to be connected to a respectivelydifferent source of signals, a common output circuit for said channels,separate means for each channel providing negative signal voltagefeedback, and a common positive signal voltage feedback path from saidcommon output circuit to each of said channels.

2. In combination with at least two signal transmission channels eachhaving input terminals adapted to be connected to a respectivelydifferent source of signals, a common output circuit for said channels,separate means for each channel providing negative signal voltagefeedback, a common positive signal voltage feedback path from saidcommon output circuit to each of said channels, and said common positivefeedback path including an impedance in said common output circuitacross which is developed the positive feedback voltage.

3. In combination with a pair of amplifier channels, each of saidchannels having input terminals upon which may be applied signals ofdifferent character, a common output circuit for said channels includinga load whose impedance is adapted to vary, separate negative signal 9voltage feedback means in each of said channels, an impedance in saidcommon output circuit adapted to develop a signal voltage whosemagnitude depends on the load impedance, and means for applying saidlast named signal voltage in regenerative phase to each of said channelstherevby to overcome the effect of the variable load impedance on thesignal output level at said common output circuit.

4. In combination with a plurality of audio frequency amplifiers,separate signal sources each feeding a respective one of the ampliers,acommon output circuit connected to the output terminals of all of saidampliers, said output circuit including a load whose impedance isadapted to vary, and separate degenerative and regenerative feedbackmeans operatively associated with at least one of said amplifiers toovercome the eiect of said variable load impedance.

5. In combination with at least two audio frequency signal transmissionchannels each having input terminals adapted to be connected to arespectively diierent source of audio signals, a common audio outputcircuit for said channels, a plurality of reproducer outlet jacks insaid output circuit, means for at least one channel providing negativesignal voltage feedback, and a positive signal voltage feedback pathfrom said common output circuit to at least said one channel.

6. In combination with at least two signal transmission channels eachhaving input terminals adapted to be connected to a respectivelydifferent source of signals, a common output circuit for said channels,means for one channel providing negative signal voltage feedback, and apositive signal voltage feedback path from said common output circuit tosaid channel, and said positive feedback path including a resistiveimpedance in said common output circuit across which is developed thepositive feedback voltage.

7. In combination with a pair of amplier channels, each of said channelshaving input terminals upon which may be applied signals of differentcharacter, a common output circuit for said channels including a loadwhose impedance is adapted to vary, negative signal voltage feedbackmeans in at least one of Said channels, a resistive impedance in saidcommon output circuit adapted to develop a signal voltage whosemagnitude depends on the load impedance, and means for applying saidlast named signal voltage in regenerative phase to said channel therebyto overcome the effect of the variable load impedance on the signaloutput level at said common output circuit.

8. In combination with at least three signal transmission channels eachhaving input terminals adapted to be connected to a respectivelydifferent source of signals, a common output circuit for said channels,means in one channel providing negative signal voltage feedback, and apositive signal voltage feedback path from said common output circuit tosaid one channel.

9. In combination with at least three signal transmission channels eachhaving input terminals adapted to be connected to a respectivelydiiierent source of signals, a common output circuit for said channels,means in one channel providing negative signal voltage feedback, apositive signal voltage feedback path from said common output circuit tosaid one channel, and said positive feedback path including an impedancein said common output lcircuit across which is developed the positivefeedback voltage.

10. In combination with at least three amplier channels, each of saidchannels having input terminals upon which may be applied signals ofdiiferent character, a common output circuit for said channels includinga load whose impedance is adapted to vary, negative signal voltagefeedback means in one of said channels, a resistor in said common outputcircuit adapted to develop aY signal voltage whose magnitude depends onthe load impedance, and means for applying said last named signalvoltage in regenerative phase to said one channel thereby to overcomethe effect of the variable load impedance on the signal output level atsaid common output circuit.

ll. In combination with three audio frequency signal transmissionchannels each having input terminals adapted to be connected to arespectively diiferent source of audio signals, a common audio outputcircuit for all of said channels, a plurality of reproducer outlet jacksin said output circuit, means for one channel providing negative signalvoltage feedback, and a positive signal voltage feedback path from saidcommon output circuit to the one channel.

CARL G. SERIGHT.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,365,575 Maxwell Dec. 19, 19442,220,770 Mayer NOV. 5, 1940 2,250,996 Mayer July 29, 1941 2,364,389Roche et al. Dec. 5, 1944 2,131,366 Black Sept. 27, 1938

